Clarithromycin for Sore Throat: The Penicillin-Allergy Option
What to take for a sore throat if you are allergic to penicillin: clarithromycin dose, course length, side effects and the interactions that matter.
Part of our Sore Throat condition guide.
Key fact: Clarithromycin is the UK alternative first choice for bacterial sore throat where penicillin cannot be used — but more than 90 per cent of people carrying a penicillin allergy label are not actually allergic, so it is always worth checking the label before defaulting to it.
Get Clarithromycin Online
Access Doctor is a UK online doctor and GPhC-registered pharmacy providing clarithromycin for bacterial sore throat following a regulated online consultation, without a GP appointment. Dispensed from our own pharmacy and delivered discreetly across the UK.
Order Clarithromycin →What is clarithromycin?
Clarithromycin is an antibiotic in the macrolide family, alongside erythromycin and azithromycin. In the UK it is a prescription-only medicine, supplied usually as 250mg or 500mg tablets, with a suspension available for children.
It has a broad range of uses beyond the throat: chest infections, skin and soft tissue infections, ear infections, and as part of combination therapy to eradicate Helicobacter pylori in stomach ulcer disease. For sore throat specifically, its role is defined and narrow — it is the alternative when penicillin is not an option.
When clarithromycin is used for sore throat
NICE guideline NG84 positions clarithromycin as the alternative first choice. It is prescribed for a bacterial sore throat when:
- You have a documented or genuine allergy to penicillin
- You cannot tolerate phenoxymethylpenicillin, for example because of severe gastrointestinal upset on a previous course
- Penicillin V is unsuitable for another clinical reason your prescriber identifies
The antibiotic question comes first. Clarithromycin is only relevant if an antibiotic is indicated at all. Most sore throats are viral and need no antibiotic of any kind — being penicillin-allergic does not change that calculation. Our guide on antibiotics for sore throat covers when treatment is justified.
How clarithromycin works
Penicillins attack the bacterial cell wall. Macrolides do something different: clarithromycin binds to the bacterial ribosome, the molecular machine that assembles proteins, and blocks it.
Without functioning ribosomes, bacteria cannot manufacture the proteins they need to grow, repair themselves or reproduce. The population stops expanding and your immune system clears what remains. This makes clarithromycin primarily bacteriostatic — it halts growth rather than directly bursting cells the way penicillin does.
Because the mechanism and the molecular structure are entirely different from penicillin, there is no cross-reactivity. Someone who reacts to penicillin has no increased likelihood of reacting to clarithromycin.
Clarithromycin dose for sore throat
| Age | Clarithromycin dose | Course length |
|---|---|---|
| Adults and 12–17 years | 250mg–500mg twice a day | 5 days |
| Children by weight | 7.5mg/kg twice a day, maximum 500mg twice a day | 5 days |
Two practical differences from penicillin V are worth noting. First, clarithromycin can be taken with or without food, so there is no need to time doses around meals. Second, twice daily is far easier to sustain than four times daily, which is a real advantage when you feel unwell.
The course for sore throat is 5 days, shorter than the 5 to 10 days used for penicillin V. As with any antibiotic, complete the course even once you feel better.
Clarithromycin vs phenoxymethylpenicillin
| Phenoxymethylpenicillin | Clarithromycin | |
|---|---|---|
| Position in UK guidance | First choice | Alternative first choice |
| Drug class | Beta-lactam (penicillin) | Macrolide |
| Mechanism | Destroys cell wall, bactericidal | Blocks protein synthesis, bacteriostatic |
| Adult dose | 500mg four times daily or 1g twice daily | 250–500mg twice daily |
| Course | 5–10 days | 5 days |
| With food? | No — empty stomach required | Either is fine |
| Spectrum | Narrow | Broader |
| Strep resistance | Essentially none | Macrolide resistance does occur |
| Drug interactions | Few | Many — significant consideration |
| Distinctive side effect | Gut upset | Metallic or altered taste |
That final column explains why penicillin V stays first line for anyone who can take it. Clarithromycin is a good antibiotic, but it is broader, carries a real resistance problem in streptococci in some regions, and interacts with a long list of common medicines.
Side effects
Common
- Nausea and vomiting
- Diarrhoea
- Indigestion and abdominal pain
- An altered, bitter or metallic taste — distinctive to this drug and harmless
- Headache
- Difficulty sleeping
The taste disturbance is the one people mention most. It can be persistent and unpleasant during the course but resolves fully once you stop, and it is not a reason to abandon treatment.
Less common but important
- QT prolongation — an effect on the heart's electrical recovery that can, rarely, cause dangerous rhythm disturbances. Risk is higher if you already have a heart rhythm problem, low potassium or magnesium, or take other QT-prolonging medicines.
- Liver effects — raised liver enzymes, and rarely hepatitis. Yellowing of the skin or eyes, dark urine or severe upper abdominal pain needs prompt review.
- Vivid dreams, confusion or anxiety — uncommon, more often reported in older people.
- Clostridioides difficile diarrhoea — as with any antibiotic, severe or bloody diarrhoea during or after a course needs medical attention.
Call 999 or go to A&E if you develop swelling of the face, lips, mouth or tongue, difficulty breathing, a widespread raised itchy rash, chest tightness, palpitations with dizziness or fainting, or severe skin blistering and peeling.
Interactions: the part that matters most
This is the single most important difference between clarithromycin and penicillin V, and the reason a prescriber will ask you to list everything you take.
Clarithromycin inhibits CYP3A4, a liver enzyme responsible for metabolising a large share of common medicines. Block the enzyme and those medicines accumulate to higher levels than intended.
| Medicine group | Examples | Concern |
|---|---|---|
| Statins | Simvastatin, atorvastatin | Raised levels increase risk of muscle damage. Statin often paused during the course. |
| Anticoagulants | Warfarin, apixaban, rivaroxaban | Increased bleeding risk. Warfarin needs INR monitoring. |
| Heart rhythm drugs | Amiodarone, sotalol, digoxin | Additive QT prolongation, or raised digoxin levels. |
| Sedatives | Midazolam, some benzodiazepines | Prolonged and deepened sedation. |
| Immunosuppressants | Ciclosporin, tacrolimus | Raised levels and toxicity. |
| Gout treatment | Colchicine | Serious, potentially fatal toxicity. |
| Antipsychotics and antidepressants | Quetiapine, citalopram | Raised levels or additive QT effects. |
| Ergot medicines | Ergotamine for migraine | Risk of severe blood vessel constriction. |
Tell your prescriber everything. That includes over-the-counter medicines, herbal products and supplements — St John's wort in particular interacts with clarithromycin. This is not a formality; the list above contains combinations that are genuinely dangerous.
Pregnancy and breastfeeding
Where a penicillin cannot be used in pregnancy, UK guidance generally prefers erythromycin over clarithromycin for sore throat, because erythromycin has a longer and more reassuring safety record in pregnancy. Clarithromycin is not absolutely contraindicated and may still be chosen in particular circumstances.
Small amounts of clarithromycin pass into breast milk. It is often considered acceptable during breastfeeding, but the decision should be made by your prescriber with your circumstances in front of them. Always declare pregnancy, possible pregnancy or breastfeeding at the point of consultation.
Are you actually allergic to penicillin?
This deserves its own section, because it affects a very large number of people.
Most labels originate in childhood: a child is given penicillin for a viral illness, develops a rash caused by the virus, and the rash is attributed to the drug. The label then follows them for life. A related scenario is the amoxicillin rash of glandular fever, which is not a true allergy at all.
What a real penicillin allergy looks like
- Symptoms starting within an hour of a dose — hives, swelling of the face or throat, wheeze, collapse
- A severe blistering or peeling skin reaction
- A reaction requiring emergency treatment or hospital admission
What usually is not
- Nausea, diarrhoea or stomach upset — these are side effects, not allergy
- A non-itchy rash appearing several days into a course during a viral illness
- A family history of penicillin allergy with no personal reaction
- "My mum said I reacted as a baby" with no further detail
If your label falls into the second group, it is worth asking your GP about formal allergy assessment. Removing an inaccurate label restores access to narrower, more effective and better-tolerated antibiotics for the rest of your life.
Order Clarithromycin for an Infected Sore Throat
If you cannot take penicillin and your symptoms suggest a bacterial infection, you can request treatment online in the UK without waiting for a GP appointment. A GPhC-registered pharmacist independent prescriber reviews your allergy history and full medicine list before prescribing.
Order Clarithromycin →Practical questions
Missed a dose? Take it when you remember, unless the next is nearly due, then skip it. Do not double up.
Alcohol? No direct interaction, but alcohol worsens the nausea clarithromycin can cause and adds to dehydration. Most people do better leaving it until the course is done.
Contraceptive pill? Clarithromycin does not reduce the effectiveness of hormonal contraception. If illness causes vomiting or significant diarrhoea, follow the missed-pill advice in your leaflet.
Still unwell after the course? Contact your prescriber rather than repeating it. See our guide on a sore throat that will not go away.
When to get urgent help
Call 999 or go to A&E for difficulty breathing or swallowing, drooling, inability to open the mouth, swelling of the face or tongue, palpitations with dizziness or fainting, or a rash that does not fade under a glass.
Contact NHS 111 or your GP the same day for a very high temperature you cannot control, symptoms worsening after three to five days, yellowing of the skin or eyes, or severe diarrhoea during or after the course.
Frequently Asked Questions
What antibiotic can I take for a sore throat if allergic to penicillin?
Clarithromycin is the recommended alternative in UK guidance for people with penicillin allergy or intolerance. Adults are usually prescribed 250mg to 500mg twice a day for 5 days. In pregnancy, erythromycin is preferred over clarithromycin. Your prescriber will ask what happened when you reacted to penicillin, because many people carrying a penicillin allergy label are not truly allergic and could safely use the more effective first-line option.
What is the clarithromycin dose for a sore throat?
For adults and young people aged 12 and over, the usual dose is 250mg to 500mg twice a day for 5 days. Children are dosed by body weight, typically 7.5mg per kilogram twice daily up to a maximum of 500mg twice daily. Take the doses roughly 12 hours apart. Clarithromycin can be taken with or without food, which makes it easier to schedule than phenoxymethylpenicillin.
How does clarithromycin work?
Clarithromycin is a macrolide antibiotic. It binds to the bacterial ribosome, the machinery bacteria use to build proteins, and blocks it. Without the ability to make proteins the bacteria cannot grow or reproduce, and the immune system clears the remaining infection. This is a different mechanism from penicillin, which attacks the bacterial cell wall, and it is why clarithromycin remains an option for people who cannot take penicillins.
What are the side effects of clarithromycin?
The most common are digestive: nausea, vomiting, diarrhoea, indigestion and abdominal pain. A distinctive and frequently reported effect is an altered or metallic taste, which is harmless and resolves when the course finishes. Headache and difficulty sleeping also occur. Less commonly clarithromycin can affect heart rhythm by prolonging the QT interval, which is why your full medicine list matters before it is prescribed.
Does clarithromycin interact with other medicines?
Yes, substantially more than penicillin does. Clarithromycin inhibits the liver enzyme CYP3A4, raising blood levels of many other drugs. Important interactions include statins such as simvastatin and atorvastatin, where the risk of muscle damage rises, warfarin and other anticoagulants, some heart rhythm medicines, certain antipsychotics and antidepressants, ciclosporin, colchicine and midazolam. Always give your prescriber a complete list of everything you take, including over-the-counter products and supplements.
Is clarithromycin safe in pregnancy?
Erythromycin is generally preferred over clarithromycin in pregnancy for sore throat where a penicillin cannot be used, because there is more reassurance data available for it. Clarithromycin is not automatically ruled out and may still be appropriate in some circumstances. If you are pregnant, trying to conceive or breastfeeding, tell your prescriber before starting any antibiotic so the choice can be made properly.
Am I really allergic to penicillin?
Possibly not. Around 10 per cent of people in the UK carry a penicillin allergy label, but on formal testing more than 90 per cent of them turn out not to be allergic. Many labels come from a childhood rash that was actually caused by the viral infection being treated. This matters because the label pushes people towards broader antibiotics with more interactions and, on average, worse outcomes. If your allergy history is vague or inherited, ask your GP about formal allergy assessment.
Completing the treatment
An antibiotic addresses the infection, but pain relief is what makes the first two days bearable. Access Doctor supplies both after a short online consultation reviewed by a GPhC-registered pharmacist independent prescriber, dispensed from our UK pharmacy with no GP appointment needed. See the full range on the sore throat treatment page.
Antibiotic · Rx
Clarithromycin 500mg
Alternative first choice for bacterial sore throat with penicillin allergy.
View product →Antibiotic · Rx
Phenoxymethylpenicillin 250mg
UK first-line option if you can take penicillin.
View product →Pain relief · P
Difflam Sore Throat Spray
Benzydamine spray that numbs the throat directly.
View product →Pain relief · P
Difflam Oral Rinse
Benzydamine gargle for broader throat coverage.
View product →References
- National Institute for Health and Care Excellence. Sore throat (acute): antimicrobial prescribing (NG84). 2018, updated. nice.org.uk
- Joint Formulary Committee. British National Formulary: clarithromycin. bnf.nice.org.uk
- NHS. Clarithromycin. nhs.uk
- NICE. Drug allergy: diagnosis and management (CG183). nice.org.uk
- NICE Clinical Knowledge Summaries. Sore throat – acute. cks.nice.org.uk
- Electronic Medicines Compendium. Clarithromycin 500mg tablets SmPC. medicines.org.uk
- Medicines and Healthcare products Regulatory Agency. Drug Safety Update: macrolides. gov.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.